1. Field of Invention
This invention relates generally to an electromagnetic flowmeter in which the excitation current for the coil of the electromagnet is a low-frequency wave and in which the core of the electromagnet has a cavity therein which serves to reduce the mass of the core without, however, significantly reducing the strength of the magnetic field produced thereby.
2. Prior Art
In an electromagnetic flowmeter, the fluid whose flow rate is to be measured is conducted through a flow tube provided with a pair of diametrically-opposed electrodes, a magnetic field mutually perpendicular to the longitudinal axis of the tube and to the electrode axis being established by an electromagnet. When the fluid intersects this field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed to actuate a recorder or indicator.
The magnetic field may be either direct or alternating in nature, for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. Though A-C operation is clearly advantageous in that the A-C flow induced signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces unwanted in-phase and quadrature components that are unrelated to flow rate and, if untreated, give rise to inaccurate indications.
To overcome these drawbacks, the U.S. patent to Mannherz et al., U.S. Pat. No. 3,783,687, discloses an electromagnetic flowmeter in which the excitation current for the electromagnetic coil is a low-frequency wave serving to produce a periodically-reversed steady state flux field, whereby unwanted in-phase and quadrature components are minimized without giving rise to polarization and galvanic effects.
In an electromagnetic flowmeter whose excitation current is a low-frequency wave which creates a low-frequency pulsatory uni-directional field, it is known, as evidenced by German petty Pat. No. 7,721,163 to establish this field transversely in a flow tube formed of an unmagnetizable metal whose interior is lined by an electrically-insulating layer, the electromagnet for this purpose having a solid core of "soft" ferromagnetic material surrounded by an excitation coil.
With an unlaminated electromagnet core such as that disclosed in the above-identified German patent and with a low-frequency excitation current applied to the coil, the resultant eddy current effects are negligible. But in order to obtain a strong output signal indicative of flow rate, it is essential that the electromagnetic field established in the flow tube of the flowmeter encompass a large region in the longitudinal direction of the tube. This requirement dictates a massive core for the electromagnet. A core of this type adds to the bulk as well as to the cost of the flowmeter and presents problems in regard to attaching the core to the flow tube.